1. Field of the Invention
The present invention relates to a bonding apparatus and method and more particularly to an apparatus and method that allows accurate calculation of the amount of deviation in imaging devices that takes images of elements to be bonded.
2. Prior Art
In an existing wire bonding apparatus, a position detection camera (called a “camera”) and a bonding arm are disposed on a bonding head. The camera is mounted on an XY table; and in order to specify the bonding points on bonding parts such as semiconductor devices and the like upon which bonding is performed, the camera takes an image of a reference pattern of the bonding parts. The bonding arm is provided with a tool that is attached to one end of the arm and performs bonding.
The camera and the tool are provided on the bonding head with the optical axis of the camera and the axial center of the tool being shifted by a fixed distance in the X and Y directions so that the tool and bonding arm do not interfere with the visual field of the camera when the camera takes images of the specific pattern of a bonding member.
Generally, the distance between the optical axis of the camera and the axial center of the tool is referred to as a “camera-tool offset amount” or simply an “offset amount”.
Since the camera determines a reference point that is used to ascertain the position to which the tool is moved, it is extremely important to ascertain exactly how far the camera is offset from the tool. However, the actual offset amount varies from instant to instant as a result of, for instance, thermal expansion of a camera holder and bonding arm caused by radiant heat from the high-temperature bonding stage. Accordingly, the offset amount must be measured and calibrated when the bonding operation is initiated, and such must be also measured and calibrated with an appropriate timing during the bonding operation.
Various methods have been proposed for the measurement and calibration of offset amounts, and Japanese Patent Application Laid-Open (Kokai) No. 2000-100858 discloses one example.
In this prior art, the tip end of a tool is brought into contact with an appropriate location of a semiconductor device or in the vicinity of a semiconductor device, and a pressure mark is formed. Next, an XY table is driven so that the bonding head is moved by an offset amount that is stored beforehand in a memory, and an image including the pressure mark is acquired by the camera. Furthermore, the positional coordinates of the center point of the pressure mark are determined by performing image processing on the image thus obtained. Then, the offset amount is measured by calculating the distance between the positional coordinates of the center point of the pressure mark and the positional coordinates of the optical axis for the X and Y directions and by adding the offset amount stored beforehand in memory to this calculated distance.
Meanwhile, in the recent method, a plurality of cameras are employed and mounted on an XY table. In such method, a camera having a higher magnification is used for positioning and recognition on, for instance, the pad side, and another camera having a lower magnification is used for positioning and recognition on, for instance, the lead side. Japanese Patent Application Laid-Open (Kokai) No. S63-236340, for instance, discloses this method. In this prior art method, high-precision bonding on the pads is performed using the camera with the higher magnification, and the images of numerous leads are processed at one time using the camera with a lower magnification. Accordingly, an efficient performance is expected.
Consequently, it is possible to use the above-described conventional offset amount measurement method in an apparatus that uses a plurality of cameras; and in such a case, the offset amounts between the individual cameras and the tool are measured separately. However, the tool becomes worn or deformed as a result of use; and it is necessary to replace the tool at a frequency of approximately once a day. Therefore, the offset amounts between the individual cameras and the tool must be measured every time the tool is replaced. Such an operation is, however, practically very difficult to execute.